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12CrMoV, 12CrMoVG, 12CrMoVR - "Brittle Steel Brothers" at high temperatures

Introduction to 12Cr1MoV series materials

Introduction to 12Cr1MoV material

12Cr1MoV belongs to pearlite heat-resistant steel and is widely used in high-temperature environments due to its excellent comprehensive properties.

• Mechanical properties:

◦ Tensile strength: ≥ 490MPa.

Yield strength: ≥ 245MPa (for steel with wall thickness ≤ 16mm). As the wall thickness increases, the yield strength requirement will be adjusted.

Elongation rate: ≥ 22%.

◦ Section shrinkage rate: ≥ 50%.

Impact energy: Depending on different standards and temperature requirements, the impact energy is generally ≥ 47J at room temperature.

Hardness: generally between 179-241HB.

• Operating temperature: It has good thermal strength and a long-term operating temperature range of 540 ℃ -580 ℃. Within this temperature range, it can maintain high strength and oxidation resistance, and can withstand certain pressures and loads. When the temperature exceeds 580 ℃, its organizational stability decreases, creep rate accelerates, and mechanical properties deteriorate significantly.

Welding performance: Due to the presence of a certain amount of alloying elements (such as Cr, Mo, V), this steel has relatively good weldability

Poor. During welding, hardened structures are prone to occur, resulting in cold cracks. Among them, a high carbon equivalent is one of the important factors leading to poor weldability. However, by selecting welding materials reasonably and developing welding processes, such as strictly controlling preheating temperature, interlayer temperature, and post weld heat treatment, welding quality can be effectively improved.

• Use of welding wire and electrode: Welding materials with similar composition to the base metal are usually selected. For example, H08CrMoVA can be used as the welding wire for argon arc welding base, and R317 can be used as the welding electrode for manual arc welding cover. These welding materials can ensure a good match between the weld metal and the base metal in terms of chemical composition and mechanical properties, improving the quality and reliability of the welded joint.

Post weld heat treatment process: Generally, high-temperature tempering treatment is used. The heating speed should not be too fast, usually controlled at 150-200 ℃/h. Heat to 720-760 ℃, and the insulation time is determined according to the thickness of the welded piece. Generally, the insulation time is 2-3 minutes per millimeter thickness, but not less than 30 minutes. The cooling rate should be controlled at 70-100 ℃/h, and air cooling can be used when cooling below 300 ℃. This heat treatment process can eliminate residual stresses in welding, improve the microstructure and properties of welded joints, enhance their stress corrosion resistance and high-temperature endurance strength.

Preheating requirements: To prevent the formation of hardened structures and cold cracks during welding, preheating is essential before welding. 12Cr1MoV steel has a high content of alloying elements, and during the welding process, the cooling rate of the weld and heat affected zone is fast, making it easy to form hardened structures such as martensite. When the preheating temperature is insufficient, this hardening tendency becomes more pronounced, and cracks will occur under the combined action of welding stress and hydrogen. Once cracks appear, it will not only seriously reduce the strength of the welded joint, but also affect the integrity and reliability of the entire structure. In harsh working environments such as high temperature and high pressure, it may cause serious safety accidents. Therefore, before welding, the welded parts need to be preheated to between 200-250 ℃, and the specific temperature should be determined comprehensively based on factors such as the thickness of the welded parts, the complexity of the structure, and the welding environment. Preheating is carried out using electric heating or flame heating, and multiple temperature measurement points are evenly arranged on the welded part to ensure uniform preheating temperature, avoid local overheating or undercooling, effectively reduce the cooling rate of the welded joint, reduce the generation of hardened structure, prevent crack occurrence, and ensure welding quality.

Performance changes after adding V element

The addition of V element to 12Cr1MoV steel has had multiple positive effects on its properties.

• Improve thermal strength: V element can form small dispersed carbides (such as VC), which have high stability at high temperatures and can effectively hinder the movement of dislocations, thereby significantly improving the high-temperature strength and creep resistance of steel. In high-temperature environments above 500 ℃, the thermal strength of 12Cr1MoV steel is significantly better than similar steel grades without V, making it more suitable for high-temperature and high-pressure working conditions.

Refining grain size: V can inhibit the growth of austenite grains in steel. During the heating process, VC particles are nailed to the grain boundaries, preventing their migration and refining the austenite grains. Fine grain structure not only improves the strength and toughness of steel, but also enhances its welding performance and reduces the tendency for embrittlement in the coarse grain zone of the welding heat affected zone.

• Improve organizational stability: The addition of V element helps stabilize the organizational structure of steel. During long-term high-temperature service, the precipitation of VC can inhibit the decomposition of pearlite into ferrite and carbides, delay the spheroidization and graphitization processes of the structure, and ensure that the steel can maintain good mechanical properties and structural stability under long-term high-temperature operation. Differences between 12Cr1MoVG and 12Cr1MoVR

• Manufacturing process:

12Cr1MoVG: As a specialized material for seamless steel pipes used in high-pressure boilers, the manufacturing process is extremely strict. Firstly, high-quality steelmaking processes are used to ensure precise control of the purity and chemical composition of the molten steel. After the perforation process, the solid round billet is processed into a hollow tube billet, and then the tube billet is further processed by hot rolling, cold rolling, cold drawing and other methods to achieve precise dimensional accuracy and good surface quality. During the entire manufacturing process, multiple non-destructive testing tests such as eddy current testing, ultrasonic testing, etc. are required to ensure that there are no defects inside the steel pipe. At the same time, special normalizing and reheating heat treatment may also be carried out to optimize the structure and performance of the steel pipe, meeting the strict requirements of high-pressure boilers for high temperature performance and reliability of the pipe material.

12Cr1MoVR: mainly used for manufacturing pressure vessel plates, its manufacturing process focuses on ensuring the comprehensive performance of steel plates. Pay attention to controlling the content of harmful elements and improving the purity of steel during the steelmaking process. By using rolling technology to process steel ingots into plates of the required thickness, the pressing amount, rolling temperature, and cooling rate are strictly controlled during the rolling process to obtain good plate shape and structural properties. There are strict requirements for the thickness tolerance, flatness and other dimensional accuracy of the board, and the thickness tolerance is generally controlled within a small range. A comprehensive quality inspection is required during the manufacturing process, including mechanical performance testing, non-destructive testing (such as ultrasonic testing for internal defects), etc., to ensure that the steel plate quality meets the relevant standards for pressure vessel manufacturing.

• Molding:

◦ 12Cr1MoVG: mainly formed as pipes, with various specifications, usually ranging from 6-762mm in outer diameter and 1-120mm in wall thickness. Pipe materials are mainly used to manufacture heating surface tubes for high-pressure boilers, such as superheater tubes, reheater tubes, and main steam pipelines. These pipes need to withstand high temperature and high pressure steam media, with extremely high requirements for their dimensional accuracy, pressure resistance, and high-temperature durability.

12Cr1MoVR: mainly formed as sheet metal, with a thickness generally ranging from 8-300mm and a width of up to 4m. These plates are mainly used to manufacture various components such as shells, heads, flanges, etc. of pressure vessels. In the forming process, it is necessary to use stamping, rolling, welding and other processes according to different component shapes and size requirements for processing, which have high requirements for the forming performance, welding performance and fatigue resistance of the sheet metal.

• Execution standards:

◦ 12Cr1MoVG: The implementation standard is GB/T 5310 "Seamless Steel Tubes for High Pressure Boilers". This standard has detailed and strict regulations on the size, shape, weight, allowable deviation, technical requirements (including chemical composition, mechanical properties, process properties, surface quality, non-destructive testing, etc.), test methods, inspection rules, as well as marking, packaging, transportation, storage, etc. of steel pipes to ensure their safe and reliable operation in high-pressure boilers.

12Cr1MoVR: The implementation standard is GB/T 713 "Steel Plates for Boilers and Pressure Vessels". This standard specifies the dimensions, shape, weight, allowable deviations, technical requirements (including chemical composition, mechanical properties, process properties, surface quality, ultrasonic testing, etc.), test methods, inspection rules, as well as markings, packaging, quality certificates, etc. of steel plates used for manufacturing boilers and pressure vessels, in order to meet the quality and safety requirements of pressure vessel manufacturing.

• Application scenarios:

12Cr1MoVG: mainly used in high-pressure boiler systems in the power industry. For example, in power plant boilers, as superheater and reheater tubes, they bear high temperature and high pressure steam, transfer heat to the working medium, and achieve energy conversion. At the same time, it is also used in the main steam pipeline to transport the high-temperature and high-pressure steam generated by the boiler to the turbine, driving the turbine to rotate and generate electricity. In these application scenarios, pipes need to have excellent high-temperature strength, oxidation resistance, and creep resistance to ensure long-term safe and stable operation.

12Cr1MoVR: widely used in the manufacturing of pressure vessels in industries such as petroleum, chemical, and energy. In petrochemical plants, reaction vessels are used for various chemical reactions and need to withstand certain pressures and temperatures; Towers are used for gas-liquid separation, distillation, and other processes, which require high strength and sealing of the plates; LNG storage tanks are used to store low-temperature liquid natural gas and have strict requirements for the low-temperature toughness and welding performance of materials. In these applications, 12Cr1MoVR sheet can meet the strength, toughness, and reliability requirements of pressure vessels under different working conditions.

On site welding requirements:

◦ 12Cr1MoVG: During on-site welding, due to the welding of pipes, it is mostly full position welding, which requires high technical proficiency from welders. During the welding process, in addition to following conventional welding techniques, it is necessary to strictly control the welding environment, such as wind speed, humidity, etc., to avoid adverse effects on the welding quality. The welding sequence should be arranged reasonably to reduce welding stress and deformation. The welding speed should be moderate. If it is too fast, the weld seam may not be full, and if it is too slow, it may cause overheating and structural deterioration. Due to the high pressure environment in which pipes are used, the quality requirements for welds are extremely high. After welding, strict non-destructive testing, such as radiographic testing, is required to meet the Level II qualification standard to ensure that there are no cracks, pores, slag inclusions, or other defects inside the weld, ensuring the safe operation of the pipeline.

◦ 12Cr1MoVR: For plate welding, strict control of welding line energy is required. Excessive linear energy can cause coarse grains in the heat affected zone, leading to a decrease in performance. When welding thick plates, a multi-layer and multi pass welding process is usually used. The welding thickness of each layer should not be too large, and the interlayer temperature should be strictly controlled. Generally, the interlayer temperature should be maintained between the preheating temperature and 250 ℃. During the welding process, attention should be paid to controlling welding deformation, and measures such as anti deformation method and rigid fixation method can be used. Non destructive testing is also required, using ultrasonic testing to meet the B-level acceptance standard. At the same time, strict testing of the mechanical properties of the welded joints, including tensile, bending, impact and other tests, is carried out to ensure the overall safety and reliability of the pressure vessel.

Key points during construction and operation process

• Construction points:

Material inspection: Before construction, strict inspection shall be carried out on the 12Cr1MoV series materials. Check the quality certification documents of the materials, including whether the chemical composition, mechanical properties, etc. meet the corresponding standard requirements. Conduct a visual inspection of the material to check for defects such as cracks, folds, and scars. For pipes, it is necessary to measure the diameter and wall thickness to ensure that the dimensions meet the design requirements; For sheet metal, it is necessary to check the thickness tolerance and flatness. At the same time, sampling and retesting can be used to verify the mechanical properties and chemical composition of the material, ensuring reliable material quality.

Welding operation: Strictly follow the established welding process specifications for operation. Welders must undergo professional training and obtain corresponding qualification certificates. During the welding process, ensure accurate control of preheating temperature, interlayer temperature, and post weld heat treatment temperature, and use appropriate temperature measuring equipment for real-time monitoring. Pay attention to the matching of welding current, voltage, and welding speed to ensure the quality of the weld seam. For important welding joints, a welding process monitoring system can be used to record welding parameters for traceability and analysis.

Assembly and installation: During the process of pipe assembly and plate splicing, it is necessary to ensure the flatness of the interface and the uniformity of the gap. For pipes, use tools such as alignment tools to ensure the concentricity of pipe openings. During the assembly process of pressure vessels, pay attention to the installation sequence and orientation of each component to ensure the overall structural accuracy of the equipment. During the installation process, it is necessary to avoid damaging the surface of the material, such as scratches, dents, etc., to prevent the formation of stress concentration sources.

• Operational points:

Temperature and pressure control: Strictly control the operating temperature and pressure within the design range during equipment operation. For high-pressure boiler systems using 12Cr1MoVG pipes, it is necessary to closely monitor changes in steam temperature and pressure to prevent overheating and overpressure operation. For pressure vessels using 12Cr1MoVR plates, it is necessary to accurately control the temperature and pressure of the internal medium according to the process requirements, in order to avoid damage to the equipment caused by excessive temperature and pressure fluctuations.

Regular inspection and maintenance: Establish a regular inspection system to conduct comprehensive inspections of equipment. Check the appearance of pipelines and containers for any abnormalities such as deformation or leakage, and inspect the welds for defects such as cracks and corrosion. Regular non-destructive testing of equipment, such as ultrasonic or radiographic testing of pipelines, wall thickness measurement and surface testing of pressure vessels, etc. Timely identify and address potential safety hazards to ensure long-term safe and stable operation of the equipment.

Corrosion protection: Take effective corrosion protection measures according to the working environment of the equipment. For pipelines and containers that come into contact with corrosive media, anti-corrosion coatings, linings, and other methods can be used for protection. Regularly evaluate and maintain the effectiveness of anti-corrosion measures, promptly repair damaged anti-corrosion coatings, and prevent material performance degradation and structural damage caused by corrosion.